1. Field of the Invention
The present invention relates to an image pickup apparatus which adjusts the focus of a focus lens employed therein by measuring the contrast of an image of a subject while moving the focus lens, determines a position at which the strength of the measured contrast is maximized on the basis of changes of the strength of the contrast and move the focus lens to the position. The present invention also relates to a focus control method provided for the image pickup apparatus and relates to a focus control program implementing the focus control method.
2. Description of the Related Art
Focus control techniques include a method for determining whether the contrast of picked-up image data acquired through a lens is high or low. This focus control method is based on the concept of assuming that an image having high contrast is an image taken in an in-focus state. This method is referred to hereafter as an imager AF (Auto Focus) method. More concretely, in accordance with the imager AF method, high-frequency components of a specific area are extracted, integrated data of the extracted high-frequency components is generated and the generated integrated data of the extracted high-frequency components is used for determining whether the contrast of the picked-up image is high or low. That is to say, the focus lens is moved to a plurality of positions in order to take the same plurality of images and, then, a luminance signal of each picked-up image is subjected to filter processing such as mainly an HPF (High Pass Filter) so as to obtain an evaluation value of the strength of the contrast of every picked-up image. In the following description, the evaluation value is also referred to as an AF value.
For example, FIG. 15 is a diagram showing a curve representing the relation between evaluation values and focus-lens positions on both sides of a just in-focus position which is defined as a position at which a subject of an image pickup operation is located in an in-focus state. A position P1 of the peak of the curve is the just in-focus position. At the just in-focus position P1, the evaluation value representing the contrast of the picked-up image of the subject of an image pickup operation is maximized. In accordance with such a focus control method, the focus operation can be carried out on the basis of only information of an image taken by the imager and a distance measurement optical system is not required in addition to an image pickup optical system. Thus, the focus control method is adopted in a number of contemporary digital cameras.
The auto focus operation adopting the imager AF method is carried out continuously. In the auto focus operation carried out continuously, the distance of a movement made by the subject of the image pickup operation at the next operation is estimated from information on distances obtained along a series of times in the past as the distances of movements made by the subject of the image pickup operation. Then, an auto focus operation aimed at surroundings of the subject of the image pickup operation is also carried out. This operation is referred to as a dynamic-body tracking AF operation. That is to say, in the dynamic-body tracking AF operation, the focus is adjusted by keeping track of the subject of the image pickup operation because the subject is moving. In the dynamic-body tracking AF operation, while the velocity of the movement made by the subject of the image pickup operation and the estimation precision are being taken into consideration, as narrow a range as possible is determined by adoption of a technique described below so that a wide range is not scanned wastefully.
For example, as shown in a diagram of FIG. 16, the limits of the scan range Rn+1(={Nn+1, Fn+1}) of the focus lens in an (n+1)th AF operation are expressed by Eqs. (4) and (5) given below:Nn+1=x−(V+d)T   (4)Fn+1=x+(V−d)T   (5)
Reference notations used in the above equations are used for denoting quantities that exist at the end of the Nth AF operation as follows. Reference notation x denotes an estimated distance between in-focus positions whereas reference notation V denotes the estimated velocity of the subject of the image pickup operation. Reference notation d denotes an estimated error of the velocity of the subject of the image pickup operation whereas reference notation T denotes an interval between two adjacent AF operations.
Japanese Patent Laid-open Nos. Hei 03-229209 and Hei 01-140190 are used in this invention specification.